This invention relates to a new and improved dampening system for use in connection with lithographic printing presses. Generally speaking, the invention relates to a new and improved dampening system of the continuous contacting type but is not necessarily limited thereto which is particularly useful in connection with lithographic printing presses. More particularly, the invention relates to a new and improved dampening system for applying fountain solution in connection with lithographic presses wherein there is a differential surface speed between the dampening roller and the plate cylinder which results in improved performance achievement in the areas of reduced ghosting hickey removal and reduced ink feedback into the dampening solution and improved printing.
It is well known that the lithographic offset printing plate is treated chemically so that there are printing and nonprinting areas so that the printing area is receptive to ink. The non-printing area, on the other hand, is hydrophilic and accepts moistening fluid. In order to achieve the desired printing, a film of moistening fluid is applied to the surface of the plate which is retained by the hydrophilic area but is repelled by the printing area in order that the printing area receives the printing ink. The non-printing area, however, is separated and isolated from the printing ink by the film of moistening fluid. In this manner only the image of the printing area is transferred to the blanket cylinder and onto the paper on which the image is printed.
Generally speaking, there are two classifications of dampening systems: contacting and noncontacting. The noncontacting type consists of the brush and spray type system wherein there is a physical gap in the path the dampening fluid takes to the plate. In such systems there is no ink feedback into the dampening system fluid.
The contact category of dampening systems has two broad sub-categories; namely, continuous systems, and conventional, or ductor type systems which utilize fabric covered or bareback form rollers. It is generally accepted that continuous dampening systems are preferred because they produce high quality printing and are relatively maintenance free because cloth or paper covered rollers are not required. On the other hand, a disadvantage of continuous systems is the problem of ghosting on the printed material.
The continuous type dampening systems may be classified into three groups, i.e., plate feed systems, ink feed systems, and combination type systems.
In the plate feed type system, dampening solution is applied directly to the plate by a series of rollers which are independent from the inking system. In such a system there is a roller in contact with a supply of dampening fluid which is transferred to a dampening form roller via an intermediate roller. The dampening form roller is in rotating contact with the plate cylinder to thereby transfer the dampening fluid. Another series of rollers including an ink form roller and a vibrating roller transfers ink to the plate cylinder.
The inker feed-type system functions so that the dampening solution is fed to one of the ink system rollers rather than to the plate. Generally, the dampening fluid is directed to the first ink form roller.
In the combination type system a separate dampening form roller is provided which applies dampening fluid to the plate. In the combination type the dampening roller is connected to the inking system by a vibrating bridge roller. A typical example of the combination system is illustrated in U.S. Pat. No. 4,290,360.
The inker feed continuous-type dampening system is widely used in the printing industry even through there are areas where improvement would be desirable. One problem with the inker feed continuous type dampening system is that the stripe setting of the vibrator roller to the first ink form roller and the first ink form roller to the plate cylinder is very critical. This setting is critical because the first ink form roller runs at essentially the same speed as the plate cylinder and is friction driven by the vibrator roller and the plate cylinder. In certain instances, e.g., sheetfed presses, there is a large gap in the plate cylinder so that there are substantial time periods during the plate cylinder revolution where the only drive for the first form roller is through frictional contact with the vibrator roller. This can be troublesome because the dampening fluid on the surface of the roller is slippery, making driving contact difficult. In addition, the slip nip between the rollers is an additional load which tends to slow down the roller. In order to insure that the form roller runs at the same speed as the vibrator roller, it is generally considered necessary to maintain a relatively large stripe between the vibrator roller and the first ink form roller. If this stripe is not heavy enough or if the plate cylinder strip is too heavy, there will be speed variations resulting in poor printing. But, on the other hand, where a heavy stripe is required to the vibrator roller, good ink transfer is interfered with which contributes to the problem of mechanical ghosting. As used herein, mechanical ghosting refers to the appearance of an unwanted phantom image in a printed area. Ghosting is a faint image of a repeat of some other printed area or a repeat of one of the regions of the same printed area.
The cause is the same in both types of mechanical ghosting, and is related to the principle which governs ink transfer from an inking roller to the plate in the press. That is, when an ink roller transfers ink to the printing plate, a mirror image of the printed area is formed on the ink roller. The boundary of this mirror image represents the dividing line between (i) those areas of the roller which contacted and transferred ink to the ink receptive image area of the plate and (ii) those areas of the inked roller which transferred no ink but received a charge of dampening fluid as a result of contacting the dampened non-image areas of the plate.
Thus, the ink form rollers (i.e. those in contact with the plate cylinder) contain areas of thinner ink films and little dampening fluid and areas of thicker ink films having much emulsified water, which produce undesired phantom images on the printed form.
Inker feed dampening systems have been in widespread use in the industry for about ten years. A typical inker feed dampening system is described and shown in U.S. Pat. No. 3,168,037.
There are certain advantages to these systems such as less maintenance in that cloth or paper coverings for rollers are not required. There is a very fast response time at the begining of a run so that very few sheets are lost due to waste and there are less demands on the operator.
In a typical inker feed dampening system the dampening solution is metered into a relatively thin film by the metering nip formed between the chrome transfer roller and the resilient metering roller. Typically, these rollers are geared together and driven at the same surface speed by a separate variable speed motor. Due to the fact that the rollers are run at the same surface speed, the thickness of the dampening fluid film at the exit of the nip is determined by the speed of the rollers, the hardness of the resilient roller, the pressure between the rollers and the viscosity of the dampening fluid.
Generally speaking, all of these factors tend to remain constant except for the speed of the rollers so that the thickness of the dampening fluid at the exit of the metering nip and the feed rate of the dampening solution is varied by using the variable speed controller to vary the speed of the transfer roller and the resilient metering roller. The metered film of dampening solution goes to the nip between the first ink form roller and the transfer roller and some dampening solution is transferred from the first form roller to the plate cylinder. The transfer roller and the metering roller run at a slower speed and a "slip nip" is formed at the juncture of the transfer roller and the first form roller.
With the above described arrangement it is generally found necessary, except on very small presses, for the metering roller to be skewed so as to provide a greater dampening fluid feed rate at the ends than at the middle of the rollers. In addition, it has been found that isopropyl alcohol must be added to the fountain solution so that it remains in a thin film rather than agglomerating into small water droplets.
Another problem with the aforesaid arrangement is that when the presses are run at high speeds the slip nip can cause emulsification of the fountain solution and ink resulting in small particles of ink being fed back and disbursed in the fountain solution which causes an effect known as tinting, which means that non-image areas on the printed sheet appear to be tinted. One solution to the problem is offered by U.S. Pat. No. 3,937,141. The solution offered by the aforesaid patent is to rearrange the rollers so that the slip nip is moved to a point where there is no ink. The chrome transfer roller is run at the same speed as the ink form roller and the resilient metering roller also functions as the fountain pan roller. In this modification, the fountain pan roller is driven by a variable speed motor which controls the fountain solution feed rate. A slip nip is, therefore, formed only where there is fountain solution present. This arrangement provides a partial solution to the emulsification or ink feedback problem but requires that the first ink form roller and the transfer roller be friction driven thus placing a greater demand on the vibrating roller. Additional maintenance problems are presented where the resilient roller is used as the fountain pan roller.
A problem with continuous systems has been ghosting. There have been several efforts to solve the ghosting problem arising in plate feed type systems.
One proposal to solve the ghosting problem is to use a separate dampening form roller but in this proposal a vibrating bridge roller is used to connect the dampening form roller to the first ink form roller.
The foregoing system has been modified in several ways. One modification was to provide a mechanism to move the vibrating bridge roller whereby the dampening system can be disconnected from the ink system during certain phases of the printing operation. Thus, the vibrating bridge roller can be moved out of engagement with the dampening form roller so as to reduce the problem of ghosting.
In summary, the prior art can be generally characterized as having the following characteristics:
1. The form roller which supplies dampening fluid to the plate is run at essentially the same surface speed as the plate cylinder.
2. The water feed rate is controlled by a separate motor drive which drives at least one of the dampening system rollers at a slower speed than the plate cylinder.
3. The initial metering of the dampening fluid into a thin metered film is caused by forming a metering nip between two rollers. Where the press is wide the rollers must be skewed so that there is a higher rate of feed at the ends of the roller.
While these prior art systems have found industry acceptance due to their fast response time, reduction of waste and less need for skilled operators, several areas remain for improvement.
One problem area involves the tendency for ghosting to occur despite prior efforts to prevent it from occurring. In addition, the prior art systems tend to have an initial high cost, and the slip nips cause emulsification which can result in ink feedback into the fountain solution which can cause tinting. Also, some prior art systems require the addition of alcohol to the fountain solution.
There is also known in the prior art an inking system (sometimes referred to as the "Delta System") which is used to reduce the accumulation of foreign particles (referred to as "hickeys") on lithographic printing plates during printing. This system is illustrated in U.S. Pat. No. 3,467,008 issued to Domotor. The Domotor patent is directed to a conventional system having one set of rollers for applying ink and another set of rollers for applying dampening solution. The dampening system in Domotor is not a continuous system and employs the conventional ductor roller.
In the sytem described in the Domotor patent, a gear drive is added to the first ink form roller so that it is driven at a lower surface speed relative to the speed of the plate cylinder and the vibrator roller. This causes two slip nips to form, one at the nip between the first ink form roller and the plate cylinder, and the second at the nip between the first ink form roller and vibrator roller. A scrubbing action occurs at the slip nip adjacent the plate cylinder which causes hickeys to be removed from the plate cylinder. The hickeys are transported along the ink train and are collected on a rider roller or in the ink fountain. This system, it has been found, substantially increases production and results in improved inking particularly in solid areas.
The Domotor system does have certain limitations. One is that the initial cost is high caused in part by designing a gear drive for the ink form roller. Another limitation is the relatively high power requirement of the drive for the ink rollers and the need for helical drive gears to eliminate gear streaks.
In addition, the system described in the Domotor patent has, heretofore, been used only with conventional ink and dampening systems. This has occured because it was thought necessary to run the dampening form roller at the same speed as the plate cylinder. It this was not done it was feared that slurring of half-tones and slurring of the trailing edges of solids would result.
Another problem that occurs is where a bridging roller is used which is run at the same speed as the dampening form roller which can result in ink emulsification and ghosting.